Known GFCI products typically include a metal oxide varistor (MOV) positioned across the power lines of the GFCI product, with the MOV providing some surge protection to the GFCI product circuitry by clamping transient voltages to acceptable levels.
An MOV is typically a non-linear resistance that is a very high resistance at below the threshold voltage and is typically modeled as an open circuit. At voltages above the threshold voltage, the resistance is nearly zero and transient power is dissipated. The amount of energy that an MOV dissipates is generally related to the size of the device, typically a disc or 14, 20, or 40 mm or the like. A larger MOV typically dissipates more energy, but take up more space, may be more costly and may require more open space around the device.
The nature of the clamping and the amount of energy that may be dissipated is determined by the size of the disc and voltage rating associated with a disc type MOV. Heretofore, GFCI products have typically been limited to handling transient voltages of 6 kV at 100 A. A need exists for GFCI products capable of sustaining greater transient conditions.
In addition, due to deregulation of local power authorities, overvoltage conditions may be more prevalent, requiring circuits to survive, for example, 240 V overvoltage conditions for a 120 V rated product. When such conditions occur, GFCI components such as the MOV in the typical GFCI may not survive. For example, a MOV in the typical GFCI operating beyond its rating at overvoltage may disintegrate, and thus such conditions may also destroy the rest of the electronics in the GFCI product. Furthermore, an MOV may fail by rupturing, exploding or igniting. Such failure conditions are potentially dangerous.
A need exists for a surge protection circuit which allows components such as a MOV to survive power conditions exceed voltage and current ratings, and thus enabling a GFCI product to survive overvoltage conditions.